Free-piston gas heater system

ABSTRACT

A reciprocating, free-piston gas heater having flow-restricting orifices is driven by a gas through a valving arrangement with proper timing.

United States Patent 1 Knoos 1 Feb. 20, 1973 [54] FREE-PISTON GAS HEATERSYSTEM [56] References Cited [75] inventor: stelllta m P. Knoos, PacificPalisades, UNITED STATES PATENTS a 283,925 8/1883 Root 417/392 [73]Assignee: Northrop Corporation, Beverly 2,271,022 1/1942 Nelson..417/392 Hills, Calif. 2,229,119 1/1941 Nichols et a1 ..417/312 x [22]Filed: June 1970 Primary Examiner-Robert M. Walker [21] Appl. No.:45,131 Attorney-William W. Rundle and Willard M. Graham [57] ABSTRACT[52] [1.8. CI ..417/392, 417/312 51 Im. Cl.....F04b 17/00 F04b 35/00F04b 21/00 A F'P l F 'P gas heater havng 9 F04) 39/06 restrictlngorlfices is driven by a gas through a valvmg 581 Field of Search..417/392, 393, 394, 395, 312 a'rangemem Pmpe' 5 Claims, 3 DrawingFigures 3 1 27 f c n 4a 5 ,6 q fl x FREE-PISTON GAS HEATER SYSTEM Thepresent invention relates to gas heaters, and more particularly, toimprovements in means for producing a continuous flow of hot gas from asupply at room temperature. It is known to produce gas heating by usingflow restrictors. In the present invention a reciprocating piston actson a supply of gas from the outlet of a check valve to intermittentlytransfer energy to the gas which is driven back and forth through therestrictors and out through an outlet. This invention is especiallyconcerned with a means for driving the piston, in order to provide anefficient, high power system capable of producing a substantial flow ofgas at l,500 to 2,000 K.

Briefly, my invention comprises the combination of a flow restrictorheater to heat a working gas, a mechanical interface between the heaterand a driver, and a valving mechanism using a driver gas to efficientlydrive the interface.

This invention will be more fully understood by reference to thefollowing detailed description of a preferred embodiment, and to theaccompanying illustrative drawings.

In the drawings,

FIG. 1 is a schematic diagram of the broad form of the presentinvention.

FIG. 2 is a pressure-distance graph showing transfer of energy from thedriver gas to the working gas in the present system.

FIG. 3 is an enlarged schematic view showing in more detail a preferredvalving arrangement for the system of FIG. 1.

FIG. 1 shows the general form of the present invention. Here a pistonmember 1 operates in a compression chamber 2, acting upon a working gasin the righthand end of chamber 2. The working gas enters this endthrough a check valve 4 supplied from a working gas inlet 5. The inletgas can be at room temperature. A first flow restrictor 6 in the end ofcompression chamber 2 leads into a second chamber 7, and a secondrestrictor 9 leads from second chamber 7 to a third chamber 10. An exitorifice 11 is connected to the outlet end of third chamber 10.

This forms a restrictor type or orifice type gas heater. As piston 1moves to the left, low pressure in the right end of compression chamber2 admits working gas through the check valve 4 into the compressionchamber 2. When piston 1 moves to the right, mechanical work is exertedon the working gas, heating it and pushing it through the firstrestrictor 6. As the reciprocating piston continues, the working gas issloshed back and forth through the restrictors and the oscillating flowis smoothed out until the gas is driven through the exit orifice 11 at asubstantially constant rate of flow, and at an elevated temperature.

The reciprocating piston 1 may be either a free piston or connected toan operating linkage, such as a connecting rod and crankshaft forexample. In the present case, it is preferred to drive the piston in aforward direction by a driver gas supplied through a valving arrangementas shown broadly in FIG. 1. A fourport valve 14 has one port connectedto a chamber inlet 15. Three other valve ports are respectivelyconnected to a driver gas supply 16, an intermediate chamber 17, and anexhaust passage l9.

The valve 14 has two operating positions. In position I, a first path isopen between the driver gas supply 16 and the intermediate chamber 17,and a separate path is open between the compression chamber 2 and theexhaust passage 19. In valve position II, a single open path is providedbetween the intermediate chamber 17 and the compression chamber 2. Thedriver gas supply 16 comprises a source of high pressure gas. The pistonmember 1 forms the mechanical interface between the driver gas and theworking gas.

The valve l4 is in position] as the piston 1 is moving to the left. Thisallows exhausting of the driver gas space on the left-hand side ofpiston 1 and filling of the intermediate chamber 17 from the driver gassupply 16. When piston 1 nears or reaches the left end of its travel,the valve 14 is forced to position II, where the piston 1 is drivenforward (to the right) by the working gas stored in the intermediatechamber 17. At or near the right end of piston travel, the valve 14returns to position I, again allowing exhaust of driver gas fromcompression chamber 2 and refilling of the intermediate chamber 17.

Any suitable valve timing or synchronizing means may be utilized, basedon piston position or pressures at different locations in the valvesystem. Preferably, the piston itself should be used in the timingcontrol of the valve, so that the valve operation is correctlysynchronized with piston operation.

A preferred valve mechanism is schematically shown in FIG. 3. A valvehousing 20 mates with and is attached to the driver gas end of thecompression chamber 2 containing a free piston 10. Piston 10 has acylindrical plug or nose portion 21 centered at its left end, thediameter of nose 21 being smaller than the main piston. A partiallyhollow, sliding valve spool 22 is carried in the right-hand end ofhousing 20, and a counter-pressure chamber 24 is to the left of thevalve spool 22. An external circumferential spool groove 25 surroundsone end of spool 22, and a series of radial operating ports 26 circlesthe hollow end of spool 22 and connects the outside of the spool to itsinterior. The interior diameter D of spool 22 is only slightly largerthan the diameter of the piston nose 21.

An exhaust port 27 (or series of ports), a supply port 29, a fillingport 30 and an inlet port 31 are provided in the valve housing 20 asshown. In the right-hand position shown, spool 22 is against aright-hand stop 32 provided by the internal diameter of the compressionchamber 2. In this position, the inlet port 31 is open to the fillingport 30 through the spool groove 25, the supply port 29 is closed, andthe exhaust port 27 is open to the spool interior. and to thecompression chamber 2 by the spool operating ports 26. At the lefthandposition, spool 22 is against a left-hand stop 34 provided by theinternal diameter of the counter-pressure chamber 24. In this leftposition, the only flow path through the valve is from the supply port29 through the operating ports 26 to the interior of the spool 22 andthe compression chamber 2.

Exhaust port 27 leads to the atmosphere. An intermediate chamber isconnected to the supply port 29 and filling port 30. A driver gas supplyline 35 is connected to the inlet port 31. A counter-pressure gas line36 is connected to the counter-pressure chamber 24, and means arepreferably provided to maintain a constant predetermined pressure inthis line 36. Such means may include a definite small flow ofcounterpressure gas in through a restrictor valve 37 and out through ableed valve 39, with the counter-pressure gas line 36 connected to apoint between the latter two valves. A pressure gage 40 may also beconnected to this same point.

Operation of the valve system of FIG. 3 will be explained with firstassuming a piston movement to the left, with the valve spool 22 beingheld in the (exhaust) position shown by pressure in the counter-pressurechamber 24. The free piston 1a has bounced back from the relatively highworking gas pressure at that time and hence is travelling rapidly. Whenthe piston nose 21 enters the interior diameter of the valve spool 22 oreven before, a back pressure builds up because of trapped gas in theannular space between the main body of the piston and the outer portionof the spool. Two results of such back pressure are (l) the valve spool22 is shifted to its left-hand position against the constant pressure incounter-pressure chamber 24, and (2) the piston la bounces back in theright-hand direction without touching the spool 22.

When piston la thus bounces to the right, driver gas from theintermediate chamber 17a is admitted through the spool operating ports26 to the piston 10. The high pressure driver gas maintains the valvespool 22 in its left-hand position and also drives the piston rapidly tothe right, to thus act on the working gas which has been admittedthrough the check valve 4 (FIG. 1) while the piston was at the leftside.

When piston 1a nears the right-hand end of its travel, the intermediatechamber 17a has been emptied to the point where the compression chamberpressure to the left of the piston has decreased sufficiently so thatcounter-pressure on the left of the valve spool 22 shifts the spool tothe right. Also, the piston will bounce back to the left against therelatively high working gas pres sure in its lowered volume. In theright-hand spool position, the intermediate chamber 17a is refilled andthe driver gas exhausts. The force of the exhaust gas pressure on thespool is less than the force of the counterpressure gas on the spool, sothat the spool remains in its right-hand position during this portion ofthe cycle. The cycle is now completed and the system continues operationin repeat fashion.

In order to start up the present system as described, a startingapparatus as shown in FIG. 3 may be used. A container 41 of a startinggas at a desired pressure is provided. In an outlet line from thiscontainer 41 is placed a control valve 42 having a vacuum line 44 orline of low pressure connected to one port and a start gas delivery line45 connected to an outlet port. The delivery line 45 is routed through apassage 46 is the compression chamber wall to a point at the interiorwhich is between the piston la and the valve spool 22. This interiorpoint is in communication with the annular space around the piston nose21 referred to before, when the piston is at the left-hand end and thespool 22 is in its right-hand position.

Before starting the operation, the control valve 42 is turned to havethe delivery line 45 connected to the low pressure line 44. Gas is thuswithdrawn from between the spool 22 and piston 10. The driver gas,working gas, and counter-pressure gas are all supplied to theirrespective inlets, and the intermediate chamber 17a therefore becomesfilled and pressurized. A certain pressure is also initially built up bythe working gas behind the restrictors 6 and 9, thus aiding in movingthe piston to its starting position and also charging the working gasvolume in the compression chamber to form a cushion for the piston onits first stroke.

At the desired moment of starting, the control valve 42 is turned fromthe vacuum position, through the pressure position, to a closedposition. A momentary surge of starting gas is thereby introduced whichis sufficient to shift the valve spool 22 to the left and the piston tothe right. This opens the supply port 29 for the driver gas in theintermediate chamber 17a and operation continues automatically.

FIG. 2 shows in a very general way the exchange of energy between thedriver gas and the working gas. The driver gas is supplied form a highpressure source which may be as much as 1,500 psi for example. It mayinitially be at room temperature. The working gas may also be initiallyat room or ambient temperature, but the initial pressure need be onlyhigh enough to insure a continuous flow outward from the delivery nozzle12, as for an air supply to a high-enthalpy wind tunnel for example. Ina high power system of the type disclosed herein, the working gas may beheated to a temperature of l,000 K or higher. The frequency ofoscillation of the free piston may be about 30 to cycles per second, forexample. Of course the end-most portions of the compression chambers canhave jacketed walls for pre-heating of the working gas and consequentcooling of the chamber walls. Further, the working gas for the input canbe taken from the output after its utilization, in a closed-circuitpath, for continued recirculation and higher temperature, with orwithout intercooling.

As further shown in FIG. 1, an alternate embodiment to eliminate thecheck valve 4 is a re-located working gas inlet 47 placed to the left ofthe original inlet. In this modification, the piston l performs its owncheck valve function when it closes the inlet 47 during the compressionstroke. Working gas will be admitted when the inlet is uncovered duringpiston travel to the left, and thus no check valve is needed.

Although the reciprocating piston could be driven in many ways, thepresent free-piston arrangement using a compressed driver gas withproper timing relation and wherein the reverse" piston stroke is a purebounce from working gas located between the piston and the first flowrestrictor, is preferred because of its simplicity and efficiency. Theenergy transfer from the pressurizing source of the driver gas to theworking gas can typically be made greater than 50 percent efficient. Thefree piston exhausts the driver gas to the atmosphere without anysubstantial entropy increase, e.g., in contrast to the case forshock-tube-devices.

The flow restrictors 6 and 9 may be simple plates having one or moreorifices. Two separate restrictors separated by the secondary chamber 7are disclosed herein, but the invention of course encompasses one, two,or three or more series restrictors. I

The piston member 1 or la disclosed herein is a preferred interfacebetween the driver and working gases, but it could be possible to use amuch larger diameter of compression chamber with diaphragm type devicein place of an actual piston. Hence a diaphragm is deemed to be anequivalent of a piston for the'purpose of defining this invention.

While in order to comply with the statute, the invention has beendescribed in language more or less specific as to structural features,it is to be understood that the invention is not limited to the specificfeatures shown, but that the means and construction herein disclosedcomprise the preferred form of putting the invention into efiect, andthe invention is therefore claimed in any of its forms or modificationswithin the legitimate and valid scope of the appended claims.

What is claimed is:

1. A gas heater system comprising:

a. a compression chamber having a working gas inlet,

a flow restrictor and an outlet;

b. a free piston member in said compression chamber, said piston beingfree to be operated upon only by gas pressures in a forward and reversedirection, said working gas inlet and said rstrictor being on the sameside of said piston;

c. intermittent driver gas supply means, including mechanical valvingmeans, connected to said compression chamber on the other side of saidpiston;

d. a separate intermediate driver gas chamber;

e. said valving means having a first position where (l) a flow path isopen between a pressurized driver gas supply and said intermediatechamber, and (2) a separate flow path is open between said compressionchamber and an exhaust outlet, and having a second position where a flowpath is open only between said intermediate chamber and said compressionchamber; and

f. synchronizing means between said piston and said valving means.

2. Apparatus in accordance with claim 1 wherein said intermediatechamber is sized to have its pressure substantially reduced bytransferring gas from it to said compression chamber.

3. A gas heater system comprising:

a. a compression chamber having a working gas inlet,

a flow restrictor and an outlet;

b. a piston member in said compression chamber, said working gas inletand said restrictor being on the same side of said piston;

c. intermittent driver gas supply means including mechanical valvingmeans, connected to said compression chamber on the other side of saidpiston;

d. said valving means comprising a valve housing,

and a sliding valve spool in said housing;

e. an intermediate chamber;

f. means connecting said valving means to a source of pressurized drivergas;

g. porting means between said driver gas source, in-

termediate chamber, valve spool and compression chamber for filling saidintermediate chamber and exhausting said compression chamber in onespool position, and for discharging said intermediate chamber into saidcompression chamber in a second spool position; and

. synchronizing means between said piston and said valving means.

4. A gas heater system comprising:

a. a compression chamber having a working gas inlet,

a flow restrictor and an outlet;

compression chamber,

b. a piston member in said said working gas inlet and said restrictorbeing on the same side of said piston;

c. intermittent driver gas supply means, including mechanical valvingmeans, connected to said compression chamber on the other side of saidpiston;

d. said valving means comprising a valve housing attached to saidcompression chamber with the interior of said housing opening into saidcompression chamber, and a valve spool sliding in said housing betweentwo positions;

e. an intermediate chamber;

f. porting means in said spool and housing for filling said intermediatechamber from a source of pressurized driver gas and exhausting saidcompression chamber in a first spool position next to said compressionchamber, and for discharging said intermediate chamber into saidcompression chamber in the other spool position away from saidcompression chamber; and

g. synchronizing means between said piston and said valving means.

5. Apparatus in accordance with claim 4 including timing meanscomprising a counter-pressure chamber behind said spool from saidcompression chamber, and means for maintaining a substantially constantcounterpressure, whereby the counter-pressure maintains said spool insaid first position during the exhaust stroke of said piston and thedriver gas pressure maintains said spool in said other position,overcoming said counterpressure, during the forward stroke of saidpiston.

1. A gas heater system comprising: a. a compression chamber having aworking gas inlet, a flow restrictor and an outlet; b. a free pistonmember in said compression chamber, said piston being free to beoperated upon only by gas pressures in a forward and reverse direction,said working gas inlet and said rstrictor being on the same side of saidpiston; c. intermittent driver gas supply means, including mechanicalvalving means, connected to said compression chamber on the other sideof said piston; d. a separate intermediate driver gas chamber; e. saidvalving means having a first position where (1) a flow path is openbetween a pressurized driver gas supply and said intermediate chamber,and (2) a separate flow path is open between said compression chamberand an exhaust outlet, and having a second position where a flow path isopen only between said intermediate chamber and said compressionchamber; and f. synchronizing means between said piston and said valvingmeans.
 1. A gas heater system comprising: a. a compression chamberhaving a working gas inlet, a flow restrictor and an outlet; b. a freepiston member in said compression chamber, said piston being free to beoperated upon only by gas pressures in a forward and reverse direction,said working gas inlet and said rstrictor being on the same side of saidpiston; c. intermittent driver gas supply means, including mechanicalvalving means, connected to said compression chamber on the other sideof said piston; d. a separate intermediate driver gas chamber; e. saidvalving means having a first position where (1) a flow path is openbetween a pressurized driver gas supply and said intermediate chamber,and (2) a separate flow path is open between said compression chamberand an exhaust outlet, and having a second position where a flow path isopen only between said intermediate chamber and said compressionchamber; and f. synchronizing means between said piston and said valvingmeans.
 2. Apparatus in accordance with claim 1 wherein said intermediatechAmber is sized to have its pressure substantially reduced bytransferring gas from it to said compression chamber.
 3. A gas heatersystem comprising: a. a compression chamber having a working gas inlet,a flow restrictor and an outlet; b. a piston member in said compressionchamber, said working gas inlet and said restrictor being on the sameside of said piston; c. intermittent driver gas supply means includingmechanical valving means, connected to said compression chamber on theother side of said piston; d. said valving means comprising a valvehousing, and a sliding valve spool in said housing; e. an intermediatechamber; f. means connecting said valving means to a source ofpressurized driver gas; g. porting means between said driver gas source,intermediate chamber, valve spool and compression chamber for fillingsaid intermediate chamber and exhausting said compression chamber in onespool position, and for discharging said intermediate chamber into saidcompression chamber in a second spool position; and h. synchronizingmeans between said piston and said valving means.
 4. A gas heater systemcomprising: a. a compression chamber having a working gas inlet, a flowrestrictor and an outlet; b. a piston member in said compressionchamber, said working gas inlet and said restrictor being on the sameside of said piston; c. intermittent driver gas supply means, includingmechanical valving means, connected to said compression chamber on theother side of said piston; d. said valving means comprising a valvehousing attached to said compression chamber with the interior of saidhousing opening into said compression chamber, and a valve spool slidingin said housing between two positions; e. an intermediate chamber; f.porting means in said spool and housing for filling said intermediatechamber from a source of pressurized driver gas and exhausting saidcompression chamber in a first spool position next to said compressionchamber, and for discharging said intermediate chamber into saidcompression chamber in the other spool position away from saidcompression chamber; and g. synchronizing means between said piston andsaid valving means.